Exterior Columns and their Reinforcement

Par. 48. - Exterior columns and their reinforcement shall be so proportioned as to withstand bending in addition to the direct load without exceeding the fiber stresses specified for beams elsewhere in this act.

Reinforced Concrete Buildings

Par. 49. - Reinforced concrete buildings may be supported by structural steel or cast iron columns, fireproofed in first class construction as provided elsewhere in this act. Brackets shall be provided to transmit the load from the floors to the columns. Such columns shall be computed as follows: -

Brackets

Par. 50. - (a) If the brackets are placed immediately below the floor the structural steel or cast iron columns shall be assumed to carry the load of all the floors above.

Par. 51. - (b) If the brackets are placed immediately above a floor the structural steel or cast iron columns shall be assumed to carry all the load above the brackets, and the floor or floors below the brackets shall be carried on reinforced concrete encasing the metal, designed in accordance with the requirements of this act, to the next bracket below or to the foundation. In this case, however, the surrounding concrete shall be so separated from the steel or cast iron as to permit the separate action of both.

Circular Hollow Columns

Par. 52. - Circular hollow steel or wrought iron columns filled with concrete shall be allowed to carry a load equal to the capacity of the metal casing plus the capacity of the concrete filling. The average unit stress in the casing shall be that specified elsewhere in this act for columns, and that in the concrete filling shall be in the same ratio to the unit stress in the casing which the modulus of elasticity of the concrete bears to that of the casing. Par. 53. - Columns with longitudinal reinforcement only shall have a steel area of not less than one per cent and not more than four per cent of the required effective area, and shall be allowed the stresses given in this act. Longitudinal reinforcement bars shall be straight and shall be secured against lateral displacement by steel ties not less than one fourth of an inch in diameter and placed not farther apart than sixteen diameters of the bars, nor more than twelve inches.

Columns Which Have Longitudinal Reinforcement

Par. 54. - Columns which have longitudinal reinforcement to an amount not less than one per cent and not more than four per cent of the effective area, and which also have hoops or spirals to an amount not less than one per cent of the volume of the enclosed core, spaced not farther apart in the clear than one sixth of the diameter of the enclosed core, and in no case more than two and one half inches, shall be allowed the stresses given in this act: provided, however, that no such column shall have a height greater than ten diameters of the enclosed core. The ends of hoops or spirals shall be united in such a way as to develop their full strength. The hoops or spirals shall be securely fastened to the longitudinal reinforcement or to approved spacers.

Par. 55. - Combination Floors. - Concrete floors with permanent blocks or forms of incombustible material with ribs of reinforced concrete between shall conform to the requirements of this act so far as they are applicable, but the blocks or forms shall not be assumed as taking stress. If a slab not less than two inches thick above the blocks or forms is cast monolithic with the rib, the rib and slab may be considered as a T section. If such construction forms a flush ceiling, or if a plastered ceiling on metal lath is suspended below the ribs, the fireproofing for such construction shall be that required for slabs.

Par. 56. - Working Stresses. - The following table gives the compressive strength in pounds per square inch which shall be assumed as the basis for design, a bag of cement weighing ninety-four pounds being assumed to measure one cubic foot in proportioning material, and the values given for aggregate to be the combined volume of fine and coarse aggregate measured separately.

Allowable Stresses

Par. 57. - In all computations allowable stresses shall be used, based as, hereinafter specified, upon assumed ultimate strengths as given above, and no concrete shall be used which, when made under laboratory conditions into test cylinders eight inches diameter and sixteen inches long and tested in compression at an age of twenty-eight days, does not show a strength at least equal to that given in the table.

Par. 58. - Concrete one year old shall be considered to have a compressive strength twenty-five per cent greater than that given in the table for concrete of the same grade and proportions.

Par. 59. - Bearing. - When compression is applied to a portion of a concrete surface of which the area is at least twice that to which the load is applied, a stress of thirty-five per cent of the compressive strength fixed by this act shall be allowed.

Par. 60. - Axial Compression. - For concentric compression on columns with longitudinal reinforcement only, twenty-two and five tenths per cent of the compressive strength fixed by this act shall be allowed.

Par. 61. - For concentric compression on columns the length of which does not exceed ten diameters of the core, with longitudinal reinforcement combined with hoops or spirals, thirty-five per cent of the compressive strength fixed by this act shall be allowed.

Par. 62. - Bending. - Compression on extreme fiber in bending shall not exceed thirty-two and five tenths per cent of the compressive strength fixed by this act: provided, however, that adjacent to the supports of continuous beams or slabs thirty-seven and five tenths per cent may be used.

Par. 63. - Shear and Diagonal Tension. - In the calculation of beams in which the maximum shearing stress in a section is used as the means of measuring the resistance to diagonal tension stress, the vertical shearing unit stress as computed by the formula v = v/ bjd' where v is the shearing unit stress, V is the total sheer, b is the breadth of the beam, and jd is the arm of the resisting couple, shall not exceed the following percentages of the respective compressive strengths fixed by this act.